Asymmetric membranes

ABSTRACT

Disclosed herein are porous asymmetric silicon membranes. The membranes are characterized by high structural stability, and as such are useful as anode components in lithium ion batteries.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application62/330,552, filed on May 2, 2016, the contents of which are herebyincorporated in its entirety.

FIELD OF THE INVENTION

The invention is generally directed to asymmetric membranes havingnanoporous and macroporous domains. The membranes are useful componentsin rechargeable batteries such as lithium ion batteries.

BACKGROUND

Rechargeable lithium ion batteries are used in a wide variety ofdifferent applications. Lithium metal can be used at the anode, however,lithium ions tends to deposit in dendritic fashion, leading to poorcolumbic efficiency. Furthermore, metallic lithium particles can breakfree from the anode and mix with the electrolyte. If the dislodgedmetallic particles contact the cathode shorting can arise. Carbonaceousanodes, which allow the reversible intercalation of lithium ions withinthe carbonaceous material, have been developed as an alternative. Themaximum amount of lithium that can be intercalated within the graphitestructure is 1 per 6 carbon atoms, yielding a specific capacity of 372mAh/g. Silicon is an attractive alternative to carbon, at least in partbecause it has a substantially higher capacity (4200 mAh/g). However,silicon anodes have not been widely adopted due to poor mechanicalstability and the large volume variation (˜300%) between lithiated andde-lithiated states. Because of the substantial volume change duringcharge/recharge cycles, traditional silicon anodes undergo rapidpulverization, thereby diminishing battery capacity. Fractured siliconcan also consume electrolyte to form a solid interphase, furtherlowering columbic efficiency. Thus, there is a need for silicon anodeswith increased structural stability. There is also a need for lithiumion batteries having silicon anodes which do not lose efficiency overcharging cycles.

SUMMARY

Disclosed herein are asymmetric membranes including both lithium storagematerials with enhanced structural stability. The membranes can be usedas an electrode in a lithium ion battery. The asymmetric membranes havea least two portions of a porous lithium storage material, for instancesilicon, tin dioxide or vanadium pentoxide. The first portion ischaracterized by a nanoporous structure that is permeable to lithiumions. The second portion is characterized by substantially larger voids(e.g., negative spaces), channels or pores which accommodate theexpanding volume of the material over charging cycles. The asymmetricmembranes can be prepared by a phase inversion of a lithium storagematerial and carbonaceous particles suspended in in a polymericsolution. The asymmetric membranes can have one or more additional skinlayers, wherein the skin layer includes an asymmetric carbonaceousporous membrane or conductive polymer.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 includes scanning electron microscope images of: 1A) uncarbonizedPAN (polyacrylonitrile)/Si; 1B) uncarbonized CA/PAN/Si; 1C) uncarbonizedCA/PAN/Si/CA; 1D) carbonized PAN/Si; 1E) carbonized CA/PAN/Si; and 1F)carbonized CA/PAN/Si/CA.

FIG. 2 includes 2A) surface area plot of CA/PAN/Si/CA used to generateslope to input into BET equation; 2B) pore size distribution ofCA/PAN/Si/CA.

FIG. 3 includes a depiction of the thermogravimetric analysis of: 3A) SiMP; 3B) carbonized PAN; 3C) carbonized PAN/Si; 3D) carbonized CA/PAN/Si;3E) carbonized CA/PAN/Si/CA.

FIG. 4 includes a depiction of the 4A) combined Raman spectra ofas-received Si MP and all carbonized membranes; 4B) combined PXRDdiffraction patterns of as-received Si MP and all carbonized membranes(note: Si:**, graphite: G).

FIG. 5 includes a depiction of 5A) cycling performance and coulombicefficiency of CA/PAN/Si/CA with specific capacity based on totalelectrode mass and mass of Si; 5B) combined cycling performance of allbatteries with specific capacity calculated using the total electrodemass; 5C) differential voltage plot of CA/PAN/Si/CA; 5D) voltage profileof CA/PAN/Si/CA; 5E) C-Rate test of CA/PAN/Si/CA; 5F) voltage profile ofall samples of the first formation cycle.

FIG. 6 includes a scanning electron microscope image of the junctionbetween a skin layer derived from the carbonized cellulose acetate andthe bottom PAN/Si layer.

FIG. 7 includes a depiction of methods by which asymmetric tin dioxidemembranes may be obtained.

FIG. 8 includes a depiction of a scanning electron microscope image ofan asymmetric tin dioxide membranes may be obtained.

FIG. 9 includes depictions of the electrochemical performance of tindioxide asymmetric membranes: 9A) Coulombic efficiency and cyclingperformance, 9B) voltage verse capacity profiles and 9C) rateperformance of SnO₂ asymmetric membrane carbonized at 500° C. for 1 hr.Note: current density=applied current/overall membrane mass.

FIG. 10A includes a depiction of methods by which asymmetric vanadiumpentoxide membranes may be obtained. Figure includes photographic (10B)and SEM images (10C) of an asymmetric vanadium pentoxide membrane.

FIG. 11 includes depictions of the electrochemical performance ofvanadium pentoxide asymmetric membranes: Electrochemistry data of V₂O₅asymmetric membrane electrodes 11A) C-rate performance, 11B) long termperformance at a current density of 100 mA g⁻¹, 11C) voltage profiles ofV₂O₅ EO-300 GH membranes at 100 mA g⁻¹ at 1st and 380th cycles, whosecapacities are normalized to their corresponding maximum capacities forconvenient polarization comparison. The standard deviation bars in 11A)were obtained from three pieces of membranes of each type. V₂O₅ EO-300CB and V₂O₅ EO-400 CB are control samples prepared using carbon blackinstead of graphene as the additives, which are annealed in air at 300and 400° C., respectively.

FIG. 12 includes a depiction of methods by which asymmetric tin dioxidemembranes may be obtained.

DETAILED DESCRIPTION

Before the present methods and systems are disclosed and described, itis to be understood that the methods and systems are not limited tospecific synthetic methods, specific components, or to particularcompositions. It is also to be understood that the terminology usedherein is for the purpose of describing particular embodiments only andis not intended to be limiting.

As used in the specification and the appended claims, the singular forms“a,” “an” and “the” include plural referents unless the context clearlydictates otherwise. Ranges may be expressed herein as from “about” oneparticular value, and/or to “about” another particular value. When sucha range is expressed, another embodiment includes¬from the oneparticular value and/or to the other particular value. Similarly, whenvalues are expressed as approximations, by use of the antecedent“about,” it will be understood that the particular value forms anotherembodiment. It will be further understood that the endpoints of each ofthe ranges are significant both in relation to the other endpoint, andindependently of the other endpoint.

“Optional” or “optionally” means that the subsequently described eventor circumstance may or may not occur, and that the description includesinstances where said event or circumstance occurs and instances where itdoes not.

Throughout the description and claims of this specification, the word“comprise” and variations of the word, such as “comprising” and“comprises,” means “including but not limited to,” and is not intendedto exclude, for example, other additives, components, integers or steps.“Exemplary” means “an example of” and is not intended to convey anindication of a preferred or ideal embodiment. “Such as” is not used ina restrictive sense, but for explanatory purposes.

Disclosed are components that can be used to perform the disclosedmethods and systems. These and other components are disclosed herein,and it is understood that when combinations, subsets, interactions,groups, etc. of these components are disclosed that while specificreference of each various individual and collective combinations andpermutation of these may not be explicitly disclosed, each isspecifically contemplated and described herein, for all methods andsystems. This applies to all aspects of this application including, butnot limited to, steps in disclosed methods. Thus, if there are a varietyof additional steps that can be performed it is understood that each ofthese additional steps can be performed with any specific embodiment orcombination of embodiments of the disclosed methods.

Disclosed herein are asymmetric porous membranes that include at leastone a lithium storage material. As used herein, the term “asymmetric”refers to membranes having different pore shapes and/or sizes across thethickness of the membrane. As used herein, a lithium storage material isa material that can intercalate and store lithium. In some embodiments,the lithium storage material can include silicon, or tin dioxide.

In addition to the lithium storage material, the asymmetric membranescan also contain amorphous carbon and/or (para) crystalline carbon. Incertain preferred embodiments, the asymmetric membranes contain silicon,amorphous carbon and (para)crystalline carbon. In certain preferredembodiments, the asymmetric membranes contain tin dioxide, amorphouscarbon and (para) crystalline carbon. In certain preferred embodiments,the asymmetric membranes contain vanadium pentoxide, amorphous carbonand (para) crystalline carbon. As used herein, amorphous carbon includesmaterials obtained from the carbonization of certain polymers, includingpolyacrylonitriles, polysulfones, cellulose acetates and polyamides. Asused herein, (para)crystalline carbon includes carbon black, fullerenes,carbon nanotubes, graphene and graphite. In certain embodiments, thelithium storage material is present in an amount from about 10-90%,about 10-80%, about 10-70%, about 10-60%, about 10-50%, about 10-40%,about 20-80%, about 25-75%, about 25-60%, about 35-60%, or about 45-55%,by weight relative to the total weight of the membrane. The lithiumstorage material content (e.g., silicon, tin dioxide or vanadiumpentoxide) can be determined by subjecting the asymmetric membrane tothermogravimetric analysis such that essentially all the carbon isoxidized and lost as CO₂ gas, whereas the lithium storage material isinert and is not lost.

The asymmetric membrane can have a thickness that is at least about 1μm, at least about 10 μm, at least about 25 μm, at least about 50 μm, atleast about 75 μm, at least about 100 μm, or at least about 200 μm. Insome embodiments, the asymmetric membrane can have a thickness fromabout 1-500 μm, about 25-400 μm, about 25-250 μm, about 50-250 μm, about75-250 μm, about 100-250 μm, about 150-250 μm, or about 100-200 μm. Insome instances, the asymmetric membrane can have a thickness that isabout 25 μm, about 50 μm, about 75 μm, about 100 μm, about 150 μm, about200 μm, about 250 μm, about 300 μm, about 400 μm, or about 500 μm.

The asymmetric membranes have at least two different zones or layers ofporous lithium storage material. The first zone is a nanoporous layerextending over the entire surface of the membrane. Generally, this layeris thin relative to the thickness of the entire membrane. The nanoporouslayer can be about 0.001-10 μm thick, about 0.01-5 μm thick, about0.05-5 μm thick, about 0.1-5 μm thick, about 0.25-5 μm thick, about0.5-5 μm thick, about 0.25-2.5 μm thick, about 0.5-2.5 μm thick, orabout 1.0-2.5 μm thick. In some embodiments, the nanopores have anaverage pore diameter of less than about 200 nm, about 150 nm, about 100nm, about 75 nm, about 50 nm, or about 25 nm. In certain embodiment, atleast 80% of the nanopores have a pore diameter less than about 200 nm,about 150 nm, about 100 nm, about 75 nm, or about 50 nm. As used herein,pore size refers average pore size which may be determined by a porosityanalyzer combined with BJH (Brunauer, Emmet, and Teller) method.

The second zone (i.e., the macroporous zone) in the asymmetric membranecontains larger negative spaces. The second zone makes up the bulk ofthe membrane; typically the thickness of the second zone is at leastabout 90%, at least about 95%, at least about 97.5%, or at least about99%, the total thickness of the asymmetric membrane. The negative spacescan be designated “macrovoids” and include macropores and macrochannels.The macrovoids have a diameter of at least about 250 nm, at least about500 nm, at least about 750 nm, at least about 1 μm, at least about 2 μm,at least about 3 μm, at least about 4 μm, at least about 5 μm, at leastabout 6 μm, at least about 7 μm, at least about 8 μm, at least about 9μm, or at least about 10 μm. When the macrovoids are macrochannels, thelength of the macrochannels is at least about 50%, about 55%, about 60%,about 65%, about 70%, about 75%, about 80%, about 85%, about 90% orabout 95%, the total thickness of the membrane. In certain embodiments,the asymmetric membrane contains two nanoporous zones with a macroporouszone disposed between them, e.g., a sandwich membrane.

It is to be understood that the two aforementioned zones are generallynot completely discrete from one another. Rather, there is a gradientalong a portion of the membrane in which the pores transition fromnanopores to macrovoids. This transition zone can have a thickness ofapproximately 0.1-5%, 0.25-5%, 0.5-5%, 0.5-2.5%, 1-2.5%, 0.1-1.0% or0.1-0.5% the total thickness of the asymmetric membrane. In someinstances, the transition zone can have a thickness of approximately1-10%, 1-7.5%, 1-5.0%, 2.5-5%, 2.5-7.5%, 5.0-7.5%, or 5.0-10% the totalthickness of the asymmetric membrane.

In some embodiments, the asymmetric membrane can include one or moreadditional skin layers. These skin layers can include asymmetricmembranes, including carbonaceous asymmetric membranes. These skinlayers can be directly adjacent to the asymmetric membrane, either onone or both sides of the asymmetric membrane, e.g., sandwich skinlayers. The carbonaceous membranes can include mixtures of amorphous and(para)crystalline carbon. In certain embodiments, the skin layers caninclude a conductive polymer. Exemplary conductive polymers include apolythiophene, a polypyrrole, a polyaniline, or polyfuran. The skinlayer(s) can be about 0.001-10 μm thick, about 0.01-5 μm thick, about0.05-5 μm thick, about 0.1-5 μm thick, about 0.25-5 μm thick, about0.5-5 μm thick, about 0.25-2.5 μm thick, about 0.5-2.5 μm thick, orabout 1.0-2.5 μm thick.

The asymmetric membranes can be characterized by a surface area of atleast about 5 m²/g, at least about 10 m²/g, at least about 15 m²/g, atleast about 20 m²/g, at least about 25 m²/g, at least about 30 m²/g, atleast about 40 m²/g, at least about 50 m²/g, at least about 60 m²/g, atleast about 70 m²/g, at least about 80 m²/g, at least about 90 m²/g, atleast about 100 m²/g, at least about 200 m²/g, at least about 300 m²/g,at least about 400 m²/g, or at least about 500 m²/g. The surface areacan be determined using a surface area analyzer combined with BET(Barrett, Joyner, and Halenda) calculation method.

The asymmetric membranes can be prepared by a phase-inversion process. Alithium storage material can be combined with polymers that aredissolved in a water soluble organic solvent. The resulting mixture canbe then be layered on a substrate at desired thickness and immersed in anon-solvent to obtain a porous film. The film can be converted to anelectrically conductive membrane by carbonizing the lithium storagematerial-polymer composite membrane in inert gases.

As used herein, the term amorphous carbon source refers to a materialthat when carbonized (e.g., heated to a temperature greater than 600°C., preferably greater than 800° C.) under an inert atmosphere, yieldsamorphous carbon. Exemplary amorphous carbon source include carbonizablepolymers such as cellulose acetates, phenolic resins,polyacrylonitriles, resorcinol-formaldehydes, polysulfones, polyamides,polyvinyls, and polyimides. Unless stated otherwise, the above mentionedpolymers may be substituted or unsubstituted.

Suitable silicon sources include silicon powders and particles, withsilicon microparticles being preferred in some embodiments. Siliconmicroparticles can have an average particle size from 0.1-10 μm, 0.1-5μm, 0.5-5 μm, or 0.5-2.5 μm. Suitable tin dioxide sources include tinoxides nanoparticles, tin alkoxides, tin acetates, tin acetoacetates,for instance SnO₂, Sn(OR)₄, Sn(acac)₂ or Sn(OC(═O)R)₄, wherein R isindependently selected from C₁₋₈ alkyl. Suitable vanadium pentoxidesources include vanadium oxides nanoparticles, vanadium alkoxides,vanadium acetates, vanadium acetoacetates, for instance VO₂, V₂O₅,V(OR)₄, V(═O)(OR)₃, V(acac)₂ or V(OC(═O)R)₄, wherein R is independentlyselected from C₁₋₈ alkyl.

Exemplary water soluble organic solvents include N-methylpyrrolidine(NMP), dimethylformamide (DMF), dimethylsulfoxide (DMSO),tetrahydrofuran (THF), diethyl ether (Et₂O), C₁₋₄ carboxylic acids,halogenated C₁₋₄ carboxylic acids, acetone, methylethyl ketone (MEK)ethyl acetate (EtOAc), and C₁₋₄ alkyl alcohols, include di and trialcohols like ethylene glycol and glycerol. The water soluble organicsolvent can include mixtures of two or more solvents.

The non-solvent can be water or any water soluble solvent that is morepolar than the water-soluble organic solvent.

The substrate can be either hydrophobic (e.g., silicon wafer orhydrophobic polymer/plastic) or hydrophilic (glass or hydrophilicpolymer, e.g., cellulose paper). Use of a hydrophobic substrate leads toan asymmetric membrane having a nanoporous zone on one side of themacrovoid zone, whereas a hydrophilic substrate can give an asymmetricmembrane having nanoporous zones on both sides of the macrovoid zone.

After the coated substrate is immersed in the non-solvent, the porousfilm can be separated from the substrate. Residual water or solvent canbe removed by immersing or washing the film with a volatile, watersoluble organic solvent followed by drying. After removal of the water,the asymmetric membrane can be formed by carbonizing the film. Exemplarycarbonization condition include heating to a temperature of at least600° C., at least 700° C., at least 800° C., at least 900° C., or atleast 1,000° C., under an inert atmosphere (e.g., He or Ar) for a periodof at least 0.5 hours, 1 hour, 2 hours, 3 hours, 4 hours or 5 hours. Incertain instances, the temperature ramp for the carbonization is nogreater than 100° C./min, no greater than 90° C./min, no greater than80° C./min, no greater than 70° C./min, no greater than 60° C./min, nogreater than 50° C./min, no greater than 40° C./min, no greater than 30°C./min, no greater than 20° C./min, or no greater than 10° C./min.

Carbonaceous skin layers can be applied to the asymmetric membrane priorto carbonization by coating or dipping the film in a mixture ofamorphous carbon or amorphous carbon source, (para)crystalline carbon,and water soluble organic solvent, followed by phase inversion in asuitable anti-solvent. Coating one layer of the film yields a singleskin on one side of the asymmetric membrane, while coating on both sides(or dipping) yields skin layers on both sides of the asymmetricmembrane. The coated film can then be carbonized to yield the double ortriple layered asymmetric membrane.

In another embodiment, after the carbonization process has beenperformed, a conductive polymer skin can be applied to one or both sidesof the membrane by either manual coating or dip coating method using aconductive polymer solution of 5-15% wt. %.

Also disclosed herein are anodes, in particular anodes for lithiumbatteries, containing asymmetric membranes. The anode includes at leasta current conductor and an asymmetric membrane affixed thereto. Ininstances of asymmetric membranes having a single nanoporous zone, thenanoporous zone is disposed on the face of the membrane facing away fromthe current collector.

The current collector can include a conductive metal, for instance aconductive metal foil. Exemplary conductive metals include copper andnickel. The asymmetric membrane can be affixed to current collector bymeans of an adhesive. In some embodiments, the adhesive can furthercontain (para)crystalline carbon. Exemplary adhesives includespolyacrylic acid, polyvinylidene fluoride, epoxy based agents, lowdensity polyethylene and the like.

The anode described herein can be advantageously employed in a lithiumion battery, either a primary or secondary battery. In addition to theanode, such battery systems can include a cathode, an electrolyte, and aseparator between the cathode and anode. The electrolyte can include atleast one lithium salt and non-aqueous solvent. Suitable lithium saltsinclude LiClO₄, LiPF₆, LiBF₄, LiAsF₆, LiSO₃F, and LiCF₃SO₃. Suitablenon-aqueous solvents include carbonates such as ethylene carbonate,dimethyl carbonate, ethyl methyl carbonate, fluoroethylene carbonate,and diethyl carbonate, and mixtures thereof. One such mixture includesethylene carbonate, dimethyl carbonate, and diethyl carbonate.Separators for lithium ion batteries are known, and typically includeone or more layers of microporous polyolefin, e.g., polyethylene and/orpolypropylene. The above components are assembled in a suitable housingknown for lithium batteries.

Lithium batteries containing the inventive anode are characterized byreduced capacity loss over charging cycles. For instance, batterieshaving anodes including asymmetric membranes exhibit no less than 70%,no less than 60%, no less than 50%, no less than 40%, no less than 30%,no less than 20%, no less than 10%, no less than 5%, no less than 2.5%,or no less than 1% capacity loss after 100 charge/recharge cycles. Theoverall capacity of such batteries, after 100 cycles, can be at least400 mAh/g, at least 450 mAh/g, at least 500 mAh/g, at least 550 mAh/g,at least 600 mAh/g, at least 650 mAh/g, at least 700 mAh/g, at least 800mAh/g, at least 900 mAh/g, or at least 1000 mAh/g.

Given the high capacity and robustness of the disclosed lithiumbatteries, said batteries may find use in high demand batteryapplications, such as electric vehicles.

EXAMPLES

The following examples are set forth below to illustrate the methods andresults according to the disclosed subject matter. These examples arenot intended to be inclusive of all aspects of the subject matterdisclosed herein, but rather to illustrate representative methods,compositions, and results. These examples are not intended to excludeequivalents and variations of the present invention, which are apparentto one skilled in the art.

Efforts have been made to ensure accuracy with respect to numbers (e.g.,amounts, temperature, etc.) but some errors and deviations should beaccounted for. Unless indicated otherwise, parts are parts by weight,temperature is in ° C. or is at ambient temperature, and pressure is ator near atmospheric. There are numerous variations and combinations ofreaction conditions, e.g., component concentrations, temperatures,pressures, and other reaction ranges and conditions that can be used tooptimize the product purity and yield obtained from the describedprocess. Only reasonable and routine experimentation will be required tooptimize such process conditions.

Example 1 Synthesis of Asymmetric Membranes

Single-layer Asymmetric Membranes

0.75 g of polyacrylonitrile (PAN) (M_(n)=150,000; Pfaltz & Bauer) wasdissolved in 11 mL N-methyl-2-pyrrolidone (NMP) (Sigma Aldrich, >99.5%)using a sonic bath (Bransonic CPX3800H) for 1 hr. Next, 0.25 g ofas-received Si powder (American Elements, ˜1 μm) and 0.20 g carbon black(TIMCAL SUPER C45 with a surface area of 45 m²/g) were added to thesolution and dispersed using a sonic bath for 2 hrs. After sonication,the homogenous solution was then coated onto a silicon (100) wafer (2in. diameter) using a doctor blade set to deliver a wet coatingthickness of 100 μm. Next, the coated wafer was immersed in deionizedwater for phase inversion. The membrane was left in DI water for 30minutes and then placed in ethanol for another 30 minutes to removeresidual moisture. Finally, the membrane was carbonized at 800° C. for 2hours in a tube furnace (Lindberg/Blue M™ 1100° C.) and under theprotection of helium gas (99.9999%, Airgas He UHP300) with a flow rateof 200 sccm. The temperature was ramped at rate of ˜60° C./min. Thesemembranes are labeled PAN/Si herein.

Double-layer Asymmetric Membranes (Single Skin Layer)

A mixture of 0.75 g of cellulose acetate (Mn=15,000; Acros) and 0.2 g ofcarbon black was dispersed in 5 mL of acetone using a sonic bath for 1hr. This mixture was then coated on the top of the un-carbonized PAN/Simembrane using a doctor blade set to ˜25 μm. Next, the coated membranewas placed into ice cold ether (ACS Grade, EMD Millipore Corporation)for phase inversion to generate a double-layer asymmetric membrane.Lastly, the membrane was carbonized at 800° C. for 2 hours in a tubefurnace and labeled as PAN/Si/CA herein.

Triple-layer Sandwich Asymmetric Membranes (Two Skin Layers)

First, 1.2 g of cellulose acetate and 0.4 g carbon black were dispersedin 15 mL acetone to make a suspension. Next, un-carbonized PAN/Siasymmetric membrane was dipped directly into the suspension and thenwithdrew slowly out of the suspension. In the next step, the dip-coatedmembrane was immersed into ice cold ether to form a triple-layersandwich asymmetric membrane. Similarly, the triple-layer sandwichasymmetric membrane was carbonized at 800° C. for 2 hours in a tubefurnace and labeled CA/PAN/Si/CA.

Control Membranes

Asymmetric membranes containing no Si MPs were prepared using theaforementioned method for control experiments. The membranes werecarbonized at 800° C. for 2 hours and labeled as PAN.

Example 2 Characterization of Asymmetric Membranes

A field emission scanning electron microscope (JEOL JSM-7600F) equippedwith a transmission electron detector (TED) was used for morphology andstructure characterizations. Raman studies were carried out using aThermoScientific DXR SmartRaman Spectrometer with a 10× lensmagnification, 150 second collection time, a 1 mW laser of 532 nm, and a50 μm slit aperture. Phase identification was performed using a powderX-ray diffractometer (Rigaku MiniFlex 600) at Armstrong StateUniversity. The samples were scanned using Cu K_(α)(λ=0.1542 nm)radiation with a step rate of 0.2°/sec from 10-90° 2Θ. The siliconcontent was determined using a thermogravimetric analyzer (TAInstruments G50 TGA). Compressed air (Ultra Zero, Airgas) with a flowrate of 20 mL/min was used as the purging gas. The temperature wasramped from room temperature to 700° C. at a rate of 10° C./min. Surfacearea and pore size distribution experiments were completed on aMicrometrics ASAP 2020 Surface Area and Porosity Analyzer. The surfacearea was calculated using Brunauer-Emmett-Teller (BET) equation and poresize distribution was determined using the Barrett-Joyner-Halenda (BJH)method. Samples were degassed at 50 μTorr for 300° C. for 30 minutes.Nitrogen adsorption and desorption was carried out at 77 K using ultrahigh purity nitrogen gas (Airgas UHP300, 99.9999%).

Because Si MPs have irregular shapes, Heywood diameter

$( {d_{p} = {2\sqrt{\frac{A}{\pi}}}} )$is used to represent the size of these particles, where A is the area ofthe particle determined from TEM images using ImageJ software. Theaverage diameter of Si MPs for this study was calculated to be ˜1.01 μmwith a standard deviation of ˜0.60 μm. SEM images reveal thatsingle-layer asymmetric membrane containing Si MPs possesses a uniqueasymmetric structure after the phase inversion process (FIG. 1a ). Theporous asymmetric structure is successfully maintained even after thehigh temperature treatment (FIG. 1d ). It can be clearly seen that largepores with a width of ˜10 μm are sandwiched between two nanoporous skinlayers (FIG. 1a ). The thickness of single-layer PAN/Si membrane wasshrunk from initial 100 μm to ˜80 μm after the phase inversion processdue to the loss of solvent after the de-mixing. After being carbonized,the PAN/Si membrane thickness was further decreased to ˜65 μm (FIG. 1d). FIGS. 1b, 1c, 1e and 1f depict asymmetric silicon membranes having orone or two skin layers. The high resolution SEM image (FIG. 6) showsthat the junction between the top carbonaceous layer derived from thecarbonized cellulose acetate and the bottom PAN/Si layer is nearlyseamless, which is beneficial to an efficient electron transfer acrossthe boundary.

The surface area and porosity were calculated using the BET equation andBJH method, respectively. The surface area of CA/PAN/Si/CA membranes is67.4 m²/g. Double layer asymmetric membranes have a surface area of 59.6m²/g. Single membranes have the lowest surface area of 36.4 m²/g. Theincreased surface area with additional carbon coating can be caused bythe high porosity of the carbonaceous layer (FIG. 2). The pore sizedistribution data shows the majority of pores are under 40 nm indiameter. The distribution also shows there are some large pores over 40nm in diameter as well.

The silicon content in asymmetric membranes was determined by using aThermogravimetric Analyzer (TGA), assuming that carbon can be fullyoxidized into CO₂ gas and Si is only slightly oxidized under our TGAconditions. It was found that there is a 99.7% mass loss for controlPAN/CB membrane that is made of pure carbon (FIG. 3b ). Under the sameTGA analytical conditions, the mass of pure Si MPs is only increased byless than 1.0%, confirming that Si can't be oxidized significantly below700° C. in air (FIG. 3a ). Based on this assumption, the contents ofsilicon by mass in PAN/Si single-layer, CA/PAN/Si double-layer andCA/PAN/Si/CA triple-layer (sandwich) membranes are 46.0%, 39.6% and34.6%, respectively (FIG. 3c-3e ). The gradual decrease in Si contentfrom single-layer to triple-layer asymmetric membranes is due to theaddition of extra carbonaceous coatings that do not contain Siparticles.

A sharp Raman peak around 520 cm⁻¹ can be seen for all samplescontaining Si MPs (FIG. 4a ), confirming the presence of cubic Si. Allsamples except for the pure Si MPs show distinct G and D peaks centeredaround 1600 cm⁻¹ and 1365 cm⁻¹, respectively, which are originated fromcarbonaceous materials in these membranes. Powder X-ray diffraction(PXRD) patterns of various carbonized membranes and pure Si MPs areshown in FIG. 4b . Characteristic patterns from cubic phase Si (111),(220), (311), (400), (331) and (422) crystal planes were observed(JCPDS-ICDD No. 27-1402). A broad pattern centered at ˜26° can be seenfrom all types of membranes, being consistent with the weak G/D peaks intheir corresponding Raman Spectra. The intensity of the pattern at 26°increases gradually from single-layer to triple-layer asymmetricmembrane due to the increasing amount of carbon as determined by TGAanalysis (FIG. 4b ).

Example 3 Electrode Preparation and Electrochemical Analysis

Asymmetric membranes were glued to a copper current collector (15 mmdiameter, 11 μm thickness; MTI Corporation) using a suspension made of0.15 g carbon black and 0.15 g of polyacrylic acid (PAA; Aldrich) in 4mL of ethanol to make the electrodes. Electrodes were then dried in anoven of 100° C. for at least 4 hours to remove residual moisture andethanol. In the next step, these dried electrodes were assembled into2032 coin cells (MTI Corporation) using Li metal as the counterelectrode and an electrolyte consisting of 1 M LiPF₆ dissolved inethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate(DEC) with a 1:1:1 v/v ratio (MTI Corporation). Apolypropylene/polyethylene membrane with pore size of 0.21×0.05 μm (MTICorporation) was used as separator. For Si MP coin cell batteries(control sample), 80 wt. % Si MPs, 10 wt. % PAA and 10 wt. % carbonblack were sonicated for 2 hours to create a homogenous slurry. Theslurry was coated onto a copper foil using a doctor blade with a wetcoating thickness of 100 μm. After being dried, the foil was punchedinto 15 mm diameter disks and then assembled into coin cells using thesame method as mentioned above. The whole battery assembly was carriedout in an argon filled glove box (LCPW, LC Technology Solutions, Inc.)with both oxygen and water content <1 ppm. Galvanostatic cycling of theLIBs was carried out on a Bio-Logic VMP3 multi-channel potentiostat witha voltage window of 0.01-1.50 V vs. Li⁺/Li. Three formation cycles at acurrent density of 100 mA/g were carried out on all coin cell batteriesbefore any other electrochemical tests.

As shown in FIG. 5b , LIB anode made from single-layer PAN/Si asymmetricmembrane has an initial capacity of 968 mAh/g, ˜62% of which capacitycan be retained after 100 cycles at a charging rate of ˜0.5 C. Incomparison, the cycling performance of pure Si MPs anode is extremelypoor as evident by nearly 100% capacity loss in as few as five cycles(FIG. 5b ). Such a dramatic difference in cyclability can be attributedto the existence of asymmetric porous structure in PAN/Si membrane,which can provide free volume for Si to expand and shrink duringalloying and de-alloying. Asymmetric silicon membranes having one ormore nanoporous skin layers exhibited reduced loss of charge capacity.FIG. 5b shows that double-layer CA/PAN/Si asymmetric membrane has a muchimproved cyclability as compared to single-layer PAN/Si asymmetricmembrane. Although there is an initial capacity loss of ˜18% in thefirst 20 cycles, the capacity maintains almost unchanged from the20^(th) to 80^(th) cycle. The initial capacity of double-layer CA/PAN/Siasymmetric membrane (852 mAh/g) is slightly lower than single-layermembrane due to a lower content of silicon. The capacity retention oftriple-layer CA/PAN/Si/CA asymmetric membrane is even higher (over 88%after 100 cycles) because both sides of the asymmetric membrane areprotected by extra porous coatings. The overall capacity of triple-layerasymmetric membrane at the 100^(th) cycle is 610 mAh/g, which is 64%larger than commercial graphite anode (372 mAh/g). The specific capacityof Si in triple-layer asymmetric membrane was calculated to be ˜1976mAh/g, assuming the capacity of carbon in the asymmetric membrane is˜340 mAh/g (FIG. 5b ).

The average coulombic efficiency of triple-layer asymmetric membrane is99.8% in 100 cycles (FIG. 5a ). When the C-rate is increased from 0.05to 0.5 C, the capacity is decreased by only ˜40%. The differentialcapacity plot of triple-layer asymmetric membrane is shown in FIG. 5c .During the first formation cycle applying a current density of 100 mA/g,there is a sharp cathodic peak at 0.05 V, which can be attributed to thelithiation of Si MPs. A sharp anodic peak at 0.46 V can be assigned tothe de-lithiation of crystalline Li_(x)Si alloys. In the followingsecond and third cycles these two peaks become broader due to theamorphorization of Si and Li_(x)Si alloys. The corresponding voltageprofiles are highly consistent with these differential capacity data(FIG. 5d ). The voltage profile of CA/PAN/Si/CA shows a long plateau ataround 0.05 V during lithiation and another long plateau can be seen at˜0.46V while being de-lithiated.

Example 4 Synthesis of SnO₂ Asymmetric Membranes

0.5 g polysulfone was dissolved in 10 mL N-methyl-2-pyrrolidone (NMP)under sonication. Then, 0.2 g carbon black and 1.0 g tin (IV)tert-butoxide were added to the polymeric solution. Next, the mixturewas coated onto a piece of silicon (100) using a doctor blade with a wetmembrane thickness of 100 μm. The coated wafer was immediately immersedinto 1 L deionized water to yield asymmetric membrane. Finally, thedried asymmetric membrane was carbonized at either 500° C. or 800° C.for one hour under the protection of high purity helium. Thede-lithiation capacity of SnO₂ asymmetric membranes carbonized at 500°C. is as high as 745 mAh g⁻¹ based on the overall membrane mass (C andSnO₂) and applying a current density of 28 mA g⁻¹. When the currentdensity is increased by 20 times from 28 to 560 mA g⁻¹, the capacity isonly decreased by 36% to 475 mAh g⁻¹. Such an excellent rate performanceis attributed to the porous networking carbon structure that facilitatesfast electron transfer and lithium ion diffusion. SnO₂ asymmetricmembrane electrodes also demonstrate an outstanding cycling performanceand Coulombic efficiency (CE), >96% retention rate in 400 cycles with anaverage CE close to 100%. The voltage profiles of SnO₂ asymmetricmembrane don't change much from the 1^(st) cycle to the 400^(th) cycle,indicating the electrode is highly stable.

Example 5 Synthesis of V₂O₅ Asymmetric Membranes

First, 0.5 g polysulfone was dissolved in 5 mL N-methyl-2-pyrrolidone(NMP) followed by adding 0.1 g graphene (GH, cheaptube.com, >98 wt. %,20-100 nm in diameter, >750 m²/g) into the polymeric solution. Then 2.0g vanadium (V) oxytriethoxide were mixed with the polymeric solutioncontaining GH by 15 min vortexing and 5 min sonication. The mixture wascoated onto a glass plate using a doctor blade with a wet thickness of150 μm. The coated glass plate was immediately immersed into deionizedwater for phase inversion and sol-gel reaction. Finally, the asymmetricmembrane was dried and carbonized at 500° C. for 1 hr under theprotection of high purity helium gas to facilitate electricalconductivity while maintaining the porous structure and then heated inair at 300° C. for 1.5 hrs to retrieve vanadium (V) oxide. Theasymmetric membrane was labeled V₂O₅ EO-300 GH herein. The membranecathodes demonstrate excellent rate performance, which can be attributedto the high surface area, nanoporous structure and conductive carboncoating on V₂O₅ nanoparticles. These cathodes delivered a capacity closeor above 200 mAh g¹ at 20 mA g¹, which is much higher than conventionalones. V₂O₅ EO-300 GH cathode demonstrates the most outstanding cyclingperformance. The capacity actually gradually increases by ˜8% throughout380 cycles, indicating that V₂O₅ can be more efficiently lithiatedduring the cycling process. It is believed that the flexible graphenenetworks surrounding V₂O₅ benefit an enhanced conductivity andstructural stability, thus allowing for repeated volume expansion andefficient lithium insertion and extraction. This protection is furtherimproved by the porous asymmetric membrane structure, resulting inexcellent capacity retention and rate performance. The normalizedvoltage profiles show that polarization in the V₂O₅ EO-300 GH cathodesis very low.

The compositions and methods of the appended claims are not limited inscope by the specific compositions and methods described herein, whichare intended as illustrations of a few aspects of the claims and anycompositions and methods that are functionally equivalent are intendedto fall within the scope of the claims. Various modifications of thecompositions and methods in addition to those shown and described hereinare intended to fall within the scope of the appended claims. Further,while only certain representative compositions and method stepsdisclosed herein are specifically described, other combinations of thecompositions and method steps also are intended to fall within the scopeof the appended claims, even if not specifically recited. Thus, acombination of steps, elements, components, or constituents may beexplicitly mentioned herein or less, however, other combinations ofsteps, elements, components, and constituents are included, even thoughnot explicitly stated. The term “comprising” and variations thereof asused herein is used synonymously with the term “including” andvariations thereof and are open, non-limiting terms. Although the terms“comprising” and “including” have been used herein to describe variousembodiments, the terms “consisting essentially of” and “consisting of”can be used in place of “comprising” and “including” to provide for morespecific embodiments of the invention and are also disclosed. Other thanin the examples, or where otherwise noted, all numbers expressingquantities of ingredients, reaction conditions, and so forth used in thespecification and claims are to be understood at the very least, and notas an attempt to limit the application of the doctrine of equivalents tothe scope of the claims, to be construed in light of the number ofsignificant digits and ordinary rounding approaches.

What is claimed is:
 1. An asymmetric membrane comprising: a) a firstportion comprising nanopores; b) a second portion, adjacent to the firstportion, comprising macrovoids; wherein the membrane has a totalthickness of at least about 1 m; wherein each portion comprisesamorphous carbon and paracrystalline carbon, wherein each portioncomprises a lithium storage material; and wherein the amorphous carboncomprises a carbonized cellulose acetate, carbonized polyacrylonitrile,carbonized polysulfone, carbonized polyamide.
 2. The membrane accordingto claim 1, wherein the lithium storage material comprises silicon. 3.The membrane according to claim 1, wherein the lithium storage materialcomprises tin dioxide.
 4. The membrane according to claim 1, wherein thelithium storage material comprises vanadium pentoxide.
 5. The membraneaccording to claim 1, wherein the paracrystalline carbon comprisescarbon black, fullerenes, nanotubes, graphite, graphene, or mixturesthereof.
 6. The membrane according to claim 1, wherein 80% of thenanopores have an average pore diameter of less than about 200 nm. 7.The membrane according to claim 1, wherein 80% of the macrovoids have adiameter of at least about 250 nm.
 8. The membrane according to claim 1,wherein the macrovoids comprise macropores.
 9. The membrane according toclaim 1, wherein the macrovoids comprise macrochannels.
 10. The membraneaccording to claim 1, wherein the thickness of the second portion is atleast about 90% the total thickness of the membrane.
 11. The membraneaccording to claim 1, wherein the membrane has a thickness of about1-500 μm.
 12. The membrane according to claim 1, wherein lithium storagematerial is present in an amount from about 10-90% by weight relative tothe total weight of the membrane.
 13. The membrane according to claim 1,further comprising at least one skin layer.
 14. The membrane accordingclaim 13, further comprising at least one skin layer immediatelyadjacent to the first portion and opposite the second portion, and atleast one skin layer immediately adjacent to the second portion andopposite the first portion.
 15. The membrane according to claim 13,wherein the skin layer comprises an asymmetric carbonaceous membrane.16. The membrane according to claim 13, wherein the skin layer comprisesa conductive polymer.
 17. An electrode for a lithium-ion batterycomprising the asymmetric membrane according to claim 1 affixed to ametal current collector.
 18. A rechargeable battery comprising: a) anelectrode comprising the asymmetric membrane according to claim 1affixed to a metal current collector; b) a counter electrode; and c) anelectrolyte.